1. Field of the Invention
The present invention relates to a television tuner. More specifically, the present invention relates to a rotary switch type VHF television tuner adapted for channel selection by means of a mechanical switching means.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a portion of a television receiver in which the present invention can be advantageously employed. A VHF television signal received by an antenna 1 is selectively amplified only with respect to a desired high frequency signal by means of an input tuning circuit or a high frequency tuning circuit 2 and the output thereof is applied to a subsequent interstage tuning circuit 3. The signal selected by the input tuning circuit 2 is further amplified by the interstage tuning circuit 3 and is applied to a frequency converter comprising a mixer 4 and a local oscillator 5. The mixer 4 of the frequency converter serves to convert the selectively received and amplified television signal into a video intermediate frequency signal by mixing the television signal with a local oscillation frequency signal obtained from the local oscillator 5. The output from the mixer 4, i.e. the video intermediate frequency signal undergoes selective amplification of a predetermined selectivity characteristic by means of a video intermediate frequency circuit 6 and the output therefrom is applied to a subsequent intermediate frequency amplifying and detecting circuit 7. A video signal obtained from the detecting circuit 7 is applied to a video circuit 8. Since this type of a VHF television tuner is well known in the art, it is not believed necessary to describe it in more detail.
A typical conventional VHF television tuner is structured using a rotary switch to implement the blocks 2, 3, 4 and 5 shown in FIG. 1. More specifically, usually such a tuner is structured without incorporating the video intermediage frequency circuit 6 therein, while such a video intermediate frequency circuit 6 is usually disposed in a main print circuit board of a television receiver. Thus, typically such a video intermediate frequency circuit is structured with three transistors, three to five tuning coils and circuit portions associated therewith and is coupled to a tuner by means of a coaxial cable. Naturally the length of such coaxial cable must be changed in accordance with models of television receivers. Accordingly, it follows that a stray capacitance due to such coaxial cable is changeable depending on the types of television receivers. Therefore, it is necessary to change the circuit constant of circuit components of a tuner and a video intermediate frequency circuit depending on the types of television receivers. Furthermore, since undesired radiation could change by moving a coaxial cable, it is necessary to consider employing some expedient to avoid such inconvenience.
Furthermore, in such a conventional television receiver a video intermediate frequency circuit portion occupies approximately a quarter of a main print circuit board. Such an arrangement is liable to adversely affect other circuit portions because of the high frequency and hence necessitates some preventive measure such as a shield or the like. In addition, since a conventional video intermediate frequency circuit has been structured with discrete inductances and capacitances, it is necessary to adjust the characteristic of the circuit. Nevertheless, it is difficult to independently adjust a video intermediate frequency circuit, without connecting a coaxial cable and a tuner. In other words, in adjusting a video intermediate frequency circuit, it is necessary to couple a tuner and a coaxial cable, as described previously.